A new model for the Warm Absorber in NGC 3783 : a single medium in total pressure equilibrium

Context. Many active galactic nuclei exhibit X-ray features typical of the highly ionized gas called “Warm Absorber” (WA). Such a material appears to be stratified, displaying zones of di fferent density, temperature, and ionization. In this paper, we investigate the possibility of modelling the WA gas in NGC 3783 as a single medium in total pressure equilibrium. Aims. Our goal is to demonstrate that the WA can be well modelled assuming constant total pressure, in contrast to the current de scriptions that are based on the presence of multiple regions, each in constant density. The assumption of total pressure equilibrium yields a more physical description of the WA, resulting in the natural stratificati on of the ionized gas, and providing an explanation for the presence of lines from different ionization states, as observed in WA spectra. Methods. We have used the photoionization code TITAN, developed by our team, to compute a grid of constant total pressure models with the purpose of fitting the WA in NGC 3783. We have compared our models to the 900 ks Chandra spectrum of NGC 3783 and to previous studies where the WA was described by multiple zones of constant density. Results. In the case of NGC 3783, the WA features can be well reproduced by a clumpy, ionized gas with cosmic abondances, ionization parameter� = 2500 erg cm s −1 , column density NH = 4 10 22 cm −2 , and constant total pressure. Conclusions. We have shown that the WA in NGC 3783 can be modelled by a single medium in total pressure equilibrium; this is probably the case for other WAs currently described by multi-zone, constant density models. In addition, our work demonstrates that the TITAN code is well adapted to the study of the WA in active galactic nuclei, opening new prospects for the use of TITAN by a larger community.